Gas separation is important in many industries and can typically be accomplished by flowing a mixture of gases over an adsorbent that preferentially adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the mixture. Fossil fuels currently supply the majority of the world's energy needs and their combustion is the largest source of anthropogenic carbon dioxide emissions. Carbon dioxide is a greenhouse gas and is believed to contribute to global climate change. Concern over global climate warming has led to interest in capturing CO2 emissions from the combustion of fossil fuels. CO2 can be removed from combustion flue gas streams by varying methods.
Combustion gases vary in composition depending on the fuel and the conditions of combustion. The combustion gases can be produced in furnaces and in gas turbines, including the combustion gases produced in the generation of electric power. The fuels used are predominantly coal and natural gas. Coal is burned in furnaces, while natural gas is burned both in furnaces and in gas turbines, but in electric power generation natural gas is mainly burned in gas turbines.
The quantities of combustion gas produced in electric power generation are very large because of the scale of furnaces and turbines used. One measure of the scale of these operations is the amount of CO2 produced in a typical 500 Megawatt power plant. For coal fired power generation, the rate of CO2 production is on the order of 100 kilograms per second; for gas fired power production it is more like 50 kilograms per second.
The challenge for flue gas CO2 capture is to do it efficiently to minimize the cost. All post-combustion CO2 capture technologies suffer from the disadvantage that the CO2 in the flue gas is present at low pressure (just about 1 atm) and in low concentrations (3 to 15%). A large amount of energy is needed to separate the CO2. For 90% recovery of 10% CO2 in a flue gas at 1 atm, the CO2 must be brought from 0.1 atm to 1 atm, and then further compressed to a delivery pressure of 150 atm. Analyses conducted at NETL shows that CO2 capture and compression using a conventional absorption process raises the cost of electricity from a newly built supercritical PC power plant by 86%, from 64 cents/kWh to 118.8 cents/kWh (Julianne M. Klara, DOE/NETL-2007/1281, Revision 1, August 2007, Exhibit 4-48 LCOE for PC Cases). Aqueous amines are considered a state-of-the-art technology for CO2 capture for PC power plants, but have a cost of $68/ton of CO2 avoided) (Klara 2007, DOE/NETL-2007/1282). Developing methods that minimize the amount of energy and other costs will be necessary if CO2 removal from flue gas is to be economical.
Methods for the removal of CO2 from gas streams, include adsorption with a solvent, adsorption with a sorbent, membrane separation, and cryogenic fractionation and combinations thereof. In absorption/desorption processes to capture CO2, the energy needed to regenerate the sorbent or solvent is a large cost element.
One of the more important gas separation techniques is temperature swing adsorption (TSA). TSA processes also rely on the fact that under pressure gases tend to be adsorbed within the pore structure of the microporous adsorbent materials or within the free volume of a polymeric material. When the temperature of the adsorbent is increased, the adsorbed gas is released, or desorbed. By cyclically swinging the temperature of adsorbent beds, TSA processes can be used to separate gases in a mixture when used with an adsorbent that is selective for one or more of the components in a gas mixture. TSA processes are generally preferred when the adsorbate concentration in the feed is less than 10%, although TSA processes can be used at greater percentages. See Jennifer Wilcox, CARBON CAPTURE, Table 4.6, 160 (Springer 2012).
Another important gas separation technique is known as a displacement purge or displacement desorption (“DD”). In the DD cycle the displacement purge fluid in the regeneration step adsorbs nearly as strongly as the adsorbate so that desorption is favored by both change in partial pressure and competitive adsorption through the displacement of surface-bound CO2. Typical cycle times are on the order of several minutes. In this process since the heat of adsorption of the displacement purge fluid, normally steam, is approximately equal to that of the adsorbate, the net heat generated or consumed is essentially negligible, maintaining nearly isothermal conditions throughout the process, which allows for higher sorbent loading compared to an inert-purge process. DD processes are generally preferred when the adsorbate concentration in the feed is greater than 10%, although DD processes are used at lesser percentages. See Jennifer Wilcox, CARBON CAPTURE, Table 4.6, 160 (Springer 2012).
Conventional swing adsorption processes suffer a variety of drawbacks. Specifically, in both TSA and DD processes, the ratio of steam required to the amount of CO2 recovered is oftentimes at an economically inefficient/unacceptable level, e.g. between 3 to 10 moles of steam usage per mole of CO2 recovered. There is a need for energy efficient removal of carbon dioxide from low pressure flue gas with existing adsorbents.
U.S. Pat. No. 8,900,347 describes a temperature swing adsorption apparatus. The apparatus includes axial thermally conductive filaments that can assist with heating and/or cooling of the adsorbent.
U.S. Pat. No. 8,784,533 describes a temperature and/or pressure swing adsorption process using a solid adsorbent, such as an adsorbent provided as a parallel channel contactor. The temperature of the solid adsorbent can be controlled by introducing a heating and/or cooling fluid through heating and/or cooling channels in the adsorbent that are not in fluid communication with the channels that provide the feed gas for separation. This can allow physical contact between the heating and/or cooling fluid without exposing the gas being separated to the fluid.
U.S. Publication No. 2015/0008366 A1 describes a displacement process an essentially isothermal cyclic adsorption process, and is incorporated herein by reference in its entirety. A driving force for adsorption and desorption/regeneration of the CO2 can be a combination of concentration swing and desorptive displacement/adsorption. During adsorption, incoming CO2 molecules adsorb onto the sorbent and also displace previously adsorbed water (adsorptive displacement or displacement adsorption), during which time the water also desorbs by concentration swing. During desorption/regeneration, the water molecules from the steam adsorb onto the adsorbent and displace the CO2 (desorptive displacement or displacement desorption). The DD process utilizes an adsorbent composed of a support and a metal compound selected from the group consisting of alkali and alkaline earth.
Thomas M. McDonald et al., Cooperative Insertion of CO2 In Diamine-Appended Metal-Organic Frameworks, 519 NATURE 303 (2015) and Thomas M. McDonald et al., Capture of Carbon Dioxide From Air and Flue Gas In the Alkylamine-Appended Metal-Organic Framework mmem-Mg2(dobpdc), 134 J. AM. CHEM. SOC. 7056 (2012) describe the use of metal organic frameworks (MOFs) in temperature swing adsorption processes.